The present invention relates generally to converting lines wherein a parent roll of towel or tissue stock is rewound and cut to rolls suitable for distribution. More particularly, the invention relates to a method and apparatus for controlling the production of such converting production lines.
Rewinders and converting lines are known in the art. Typically converting lines including one or more rewinders also include a log accumulator, one or more logsaws, one or more wrappers and one or more casepackers with associated tail sealers, convergers, diverters, conveyers and like equipment. Typically a converting operation for tissue or towel to form rolls suitable for consumption will include rewinding a parent roll of material into smaller xe2x80x9clogsxe2x80x9d of diameter suitable for distribution; cutting the logs into suitable roll lengths; wrapping the rolls and packing them into cases for shipment. During rewinding, the material may be embossed or printed if so desired. The machinery included as well as the various steps involved are relatively complex, requiring sophisticated control.
There is disclosed, for example, in U.S. Pat. No. 6,168,679 to Biagiotti a method of controlling a rewinding operation by way of monitoring the diameter of a roll by correlating the angular velocity of the roll with the linear speed of the web. The information thus obtained may be used for determining when to glue the tail to the rewound log or to start a new log.
In U.S. Pat. No. 6,050,519 also to Biagiotti there is disclosed a method of converting web material based on the idea of combining the winding and gluing of the free tail edge of the log in a single section of the processing line, eliminating the accumulator as well as a station for unwinding and positioning the free tail edge of the log. The object of such a layout is to minimize the size of the converting line as well as simplify control thereof.
In U.S. Pat. No. 4,328,931 to Bullock et al. there is disclosed a method of controlling the web speed of a converting rewinder based on the status of equipment in the system.
It should further be noted that during a converting operation, the web may be calendered or embossed or both as noted in U.S. Pat. No. 5,904,812 to Salman et al. or as described in U.S. Pat. No. 5,091,032 to Schulz. The disclosure of the foregoing patents is incorporated herein by reference.
The present invention provides a way of balancing material flow, minimizing undesirable starts/stops and generally increasing the efficiency of a converting operation through an integrated control methodology and architecture. There is provided in accordance with the present invention a method of controlling a converting line provided with a rewinder, an accumulator and equipment downstream from the accumulator, including the steps of: (a) providing an upper target limit for the inventory level of the accumulator; (b) providing a lower target limit for the inventory level of the accumulator; (c) determining the slowest maximum speed of the downstream equipment; (d) determining the fastest minimum speed of the downstream equipment; (e) monitoring the average production speed of the rewinder; (f) monitoring the actual inventory level of the accumulator; (g) generating a master speed reference value which: (i) corresponds to the slowest maximum speed of said downstream equipment if the actual inventory level of said accumulator is greater than about the upper target limit for the inventory level of the accumulator; or (ii) corresponds to the fastest minimum speed of the downstream equipment if the actual inventory level of the accumulator is less than about the lower target limit for the inventory level of the accumulator; or (iii) is based on the actual production speed of the rewinder if the actual level of inventory in the accumulator is less than about the upper target limit for the inventory level of the accumulator and greater than about the lower target limit for the inventory level of the accumulator; and finally, (h) controlling the production speed of the downstream equipment based on the master speed reference value. The downstream equipment typically comprises one or more logsaws downstream of the accumulator for cutting rewound logs of tissue or towel into lengths suitable for distribution and one or more wrapping stations for applying packaging to the cut rolls. Preferably, the maximum collective production speed of the wrapping stations is capable of meeting or exceeding the maximum production speed of the rewinder and the maximum collective production speed of the logsaws.
Typically, the converting line comprises a plurality of conveyer legs characterized in that each downstream section thereof operates at a speed faster than or slower than its corresponding upstream section. So also, the converting line is usually provided with at least one conveying diverter having more output legs than input legs and at least one conveying converger having more input legs than output legs. In a preferred embodiment, the converting line has at least one diverter or converger operating in a dynamic mode with infinitely variable release ratios based on the demand of downstream equipment supplied by the diverter or converger.
Most preferably, practice of the invention will involve setting a minimum value for the rewinder speed to provide for web breaks and minor disturbances at the rewinder, as well as providing a preferred target value for the inventory level of said accumulator. In the latter cases, the master speed reference value is preferably based on the actual rewinder speed and a trim value calculated to bias the inventory of the accumulator to the preferred target value for the inventory level of the accumulator. The master speed reference value may correspond to the actual rewinder average speed summed with a trim value calculated to bias the inventory of the accumulator to the preferred target value for the inventory level of the accumulator, if so desired. Generally speaking, master speed reference value is refreshed at time intervals of two minutes or less; typically, at time intervals of 1 minute or less; and preferably at time intervals of 30 seconds or less.
The foregoing method is advantageously practiced in an integrated electronic system which stores, calculates and utilizes the control values as described and claimed below.